1. Field of Invention
The field of the currently claimed embodiments of this invention relates to force and torque sensors, and more particularly to magnetic resonance imaging (MRI) compatible, integrated force and torque sensors.
2. Discussion of Related Art
In the last decade, robot assisted MRI-guided prostate needle placement has gained a lot of interest due to the superior imaging capabilities of MRI systems. An MRI-compatible, integrated force and torque sensor could be an important part of robotic systems for prostate percutaneous interventions under MRI, for example.
An example of teleoperated needle steering under real-time MRI guidance is provided in [1]. The clinician interacts with a master robot which is placed next to the scanner and therefore must be MRI-compatible. The slave robot which is installed on the base robot [2], [3] (the base robot orients the needle toward the target), follows the motions of the master. (See also US 2012/0265051 A1, the entire contents of which are incorporated herein by reference.)
Slave and master have two degrees-of-freedom (2 DOF) each, linear motion for the needle plus the rotation of it. Since the needle is beveled, the combination of these two motions enables needle steering. Visual feedback is provided by real-time MRI. A controller enables communication between master and slave. The idea of teleoperation is to enable the surgeon to remotely perform the task while the patient is inside the scanner for real-time imaging.
Since the master robot uses piezo motors for the sake of MRI-compatibility and ease of control, it is non-backdrivable. Therefore, a 2 DOF force-torque sensor is required to enable movement. In addition, for feedback of the needle insertion force to the clinician's hand, a force sensor should be placed at the slave side behind the needle. Both force sensors should be MRI-compatible. MRI-compatibility for sensors means that: 1) the sensor should be able to operate in a high magnetic field and 2) it has minimal disturbance to the MRI images. This means that usage of any ferromagnetic parts should be reduced to a minimum, and non-magnetic metallic parts should be avoided. In addition, it implies that the conventional strain gauges should be avoided since they distort the magnetic field and the RF pulse emitted by the scanner thus drastically degrading the image quality. Shielding of strain gauges does not resolve the image degradation.
In previous studies, hydrostatic pressure, differential light intensity, differential optic fiber, optical micrometry, and absolute light intensity have been proposed as alternatives to the conventional strain gauges [4]. Tada et al. [5] developed an optical t-axis force sensor without any metal and electronic components in the sensing element using photo sensors and optical fibers. The sensor was a 2DOF sensor which measures forces in x and y directions and it was rather bulky since it used photo sensors.
Chapuis et al. [6] designed a simple and efficient torque sensor based on light intensity measurement over optical fibers. This sensor allowed the electronic components to be placed outside the scanner room. The sensitivity of transverse torque was reduced to 0.03% of the desired output torque by using a self-guiding flexible structure and optimal mirror placement. This sensor was a single DOF sensor for torque sensing and utilized optical technology for compactness and sensitivity to precise placement of the mirrors.
A MRI-compatible three-axis force sensor was developed in [7]. Differential measure of light intensity is used to develop this sensor using a new MRI-compatible optical micrometry. Optoelectronic devices and pairs of fiber optics are used to measure forces in three directions. Two micrometers were aligned in orthogonal directions in order to realize three-axis force sensitivity.
A parallel plate structure was used to develop a MRI-compatible optical force sensor. It utilized optical micrometry based on differential measures of light intensity [8]. The sensor's head component was made of glass fiber reinforced poly-ether-ether-ketone to reduce axial interference and hysteresis behavior of plastic resin. This was a 1DOF sensor for axial force sensing.
An optical fiber sensing method was designed and fabricated in [9]. The sensor was based on an optical sensing principle and measured forces by deforming a 3 DOF flexible structure. By using an optical sensing scheme, minute deflection of the structure was detected and then the magnitude and direction of the applied force was determined. This sensor measured only forces, and it was bulky because of the sensing principal.
Polygerinos et al. in [10] presented a prototype design and development of a small MRI-compatible fiber optic force sensor for force sensing during MRI-guided cardiac catheterization. A fiber optic cable interrogated a reflective surface at a predefined distance inside a catheter shaft based on light intensity modulation.
Tan et al. have developed a 3DOF axial force sensor with small coupling effects between DOFs. The use of materials with hysteretic behavior made it necessary to use Prandtl-Ishlinskii theory to address this behavior. The sensor was very bulky [11].
Hao Su et al. [4] developed a sensor for two DOF torque measurement and one DOF force measurement. This sensor is based on fiber optic and spherical mirror technology. The sensor was bulky and was not able to measure axial torque which is necessary for needle steering in prostate intervention.
Iordachita et al. designed and analyzed a force measurement device that measures distal forces interior to the sclera using FBGs embedded in a 0.5 mm diameter tool shaft. Utilizing FGB technology in this design made the device as small as possible which was necessary for retinal microsurgery. This device is can be used for force sensing in 2 directions, especially for microsurgery procedures [12].
Despite the advances reported recently concerning MRI-compatible systems, a useful MRI-compatible sensor is still a challenging task. Therefore, there remains a need for improved MRI-compatible, integrated force and torque sensors.